Are Humans Still Evolving? Are We Perhaps Devolving?
Potentially deleterious genes are not being selected against
Some people think that thanks to the wonders of modern medicine, welfare programs, and the unprecedented safety and security enjoyed by most, deleterious genes are not being selected against. In other words, whatever the genetic variability that partly underlies their varied phenotypic traits (such as body strength, alertness, diligence, prosocial behavior, and cognitive ability), most people reach reproductive age unscathed, and thus natural selection has nothing to select from — hence no further evolution.
Nevertheless, as long as people have different numbers of children, which is of course the case, evolution can’t be at rest. This presupposes, naturally, that genetic variation is part of the reason why we observe different reproductive rates among people. This is not an unfounded assumption. Indeed, that all human behavioral traits are heritable is known as the First Law of Behavior Genetics (heritability is the amount of trait variation in a population that is attributable to individual genetic differences). And even if we all had the same number of children, assortative mating alone would induce the genetic enhancement of whatever traits are favored by people when they choose their mates.
Deleterious genes
We have to bear in mind that, strictly from an evolutionary standpoint, mutations that might be considered deleterious are not technically so if they aren’t diminishing the prospects of their human hosts to survive and reproduce. They might negatively affect our fitness — as the term is commonly used (being fast, strong, smart, etc.) — but in the biological definition of the term, which is simply related to the differential success of gene variations to persist in the gene pool, they are not necessarily deleterious.
The extinct dodo provides a wonderful example of what fitness means in evolution. It was an “absurd” bird from a conventional-fitness perspective: slow, clumsy and flightless; yet it was perfectly adapted (evolutionarily fit) to its predator-free environment (before humans arrived).
In the case of humans, shortsightedness is a good example of a genetic predisposition that we might consider deleterious; but if thanks to eyeglasses, surgery, or contact lenses shortsighted people can lead perfectly normal lives, it’s not evolutionarily deleterious.
What about intelligence? IQs have even been rising. This is called the Flynn effect, according to which IQs have been increasing by about 2.5 points per decade for many decades. It’s very dubious, though, that we’re so much smarter than our recent ancestors. The Flynn effect doesn’t seem to reflect an increase in raw mental power. It’s probably just the result of more education and the higher emphasis on abstract thinking prevalent in modern times — the type of thinking that boosts IQ performance. One has to wonder, actually, how well today’s babies would fare after childhood if they were magically transported to ancient hunter-gatherer times, harsh and unforgiving times when presumably one had to be reasonably sharp to survive. Also, a negative correlation between IQ and fertility has been observed over many decades, so, given its high heritability, intelligence may be decreasing just because of that, whatever the Flynn effect says.
Now, when people talk about deleterious genes, they may simply have in mind mutations that diminish certain human traits that we value, whether or not they affect our evolutionary fitness (our chances of survival and reproduction), and regret that such mutations are not being eliminated from the gene pool thanks to the relatively easy conditions of modern life. Some may even think that their not being selected against by natural selection poses a sort of existential risk to the future of mankind. That’s theoretically possible, but I doubt it’s something to be concerned about, at least given our current circumstances.
To return to the question of whether we are still evolving: in addition to the reasons given above, the very fact that “pseudo-deleterious” gene variations are not being eliminated means that we’re evolving on that account alone, although not in a direction that we’d typically equate with improvement. For example, if poor eyesight carries no disadvantage whatsoever, we’ll all eventually evolve to have poor eyesight (excellent vision requires continuous selective pressure in order to persist). If in the future we develop bionic vision, we might evolve to become genetically blind.
Absolute technological dependence
A technological collapse today would cause untold misery and death, but humanity would certainly not go extinct as a result. We can easily imagine, however, a distant future in which we’ve become absolutely dependent on highly sophisticated technology for our survival and reproduction. If that technology collapsed for some reason, then the otherwise deleterious mutations accumulated until then (i.e., the mutations that were neutral only thanks to the technology) would instantly become truly deleterious and we’d possibly become extinct. A problem for future generations to worry about.
